Extruding mesh with holes creates weird artifacts
Your problem is that you have ngons that create this kind of artefacts. Also you have a lot of useless edges, if you use a Subdivision Surfaces modifier you won't need so many.

If I were you I would redo the object, it would take less time than correct your version:


Keep the profile only, mirror it, use the Checker Deselect and the Dissolve Vertices tool to keep only the necessary amount of vertices.





Now extrude the profile, give it some edge loops and bevel where the cut are supposed to happen, then delete the faces.





Give it some additional edge loops to make the holes square, give your object a Solidify modifier to give it thickness, keep a copy of the object somewhere, apply the Solidify modifier, bring some additional edge loops to sharp the edges again.